1. Field of the Invention
The present invention is directed to particulate substrates coated with a resin comprising phenolic-aldehyde polymer or other suitable polymer. Depending upon the resin selected, the substrate selected and how the resin is combined with the substrate, the resulting resin coated particle is useful in either subterranean formations as a curable proppant or a precured proppant. The present invention also relates to methods of making or using the resins or coated substrates.
2. Description of Background Art
The use of phenolic resin coated proppants is disclosed by U.S. Pat. No. 5,218,038 to Johnson et al (the disclosure of which is incorporated by reference in its entirety). In general, proppants are extremely useful to keep open fractures imposed by hydraulic fracturng upon a subterranean formation, e.g., an oil or gas bearing strata Typically, the fracturing is desired in the subterranean formation to increase oil or gas production. Fracturing is caused by injecting a viscous fracturing fluid or a foam at high pressure into the well to form fractures. As the fracture is formed, a particulate material, referred to as a xe2x80x9cpropping agentxe2x80x9d or xe2x80x9cproppantxe2x80x9d is placed in the formation to maintain the fracture in a propped condition when the injection pressure is released. As the fracture forms, the proppants are carried into the well by suspending them in additional fluid or foam to fill the fracture with a slurry of proppant in the fluid or foam. Upon release of the pressure, the proppants form a pack which serves to hold open the fractures. The goal of using proppants is to increase production of oil and/or gas by providing a highly conductive channel in the formation. Choosing a proppant is critical to the success of well stimulation.
The propped fracture thus provides a highly conductive channel in the formation. The degree of stimulation afforded by the hydraulic fracture treatment is largely dependent upon formation parameters, the fracture""s permeability and the fracture""s propped width. If the proppant is an uncoated substrate and is subjected to high stresses existing in a gas/oil well, the substrate may be crushed to produce fines of crushed proppant. Fines will subsequently reduce conductivity within the proppant pack. However, a resin coating will enhance crush resistance of a coated particle above that of the substrate alone.
Known resins used in resin coated proppants include epoxy, furan, phenolic resins and combinations of these resins. The resins are from about 1 to about 8 percent by weight of the total coated particle. The particulate substrate may be sand, ceramics, or other particulate substrate and has a particle size in the range of USA Standard Testing screen numbers from about 8 to about 100 (i.e. screen openings of about 0.0937 inch to about 0.0059 inch).
Resin coated proppants come in two types: precured and curable. Precured resin coated proppants comprise a substrate coated with a resin which has been significantly crosslinked. The resin coating of the precured proppants provides crush resistance to the substrate. Since the resin coating is already cured before it is introduced into the well, even under high pressure and temperature conditions, the proppant does not agglomerate. Such precured resin coated proppants are typically held in the well by the stress surrounding them. In some hydraulic fracturing circumstances, the precured proppants in the well would flow back from the fracture, especially during clean up or production in oil and gas wells. Some of the proppant can be transported out of the fractured zones and into the well bore by fluids produced from the well. This transportation is known as flow-back.
Flowing back of proppant from the fracture is undesirable and has been controlled to an extent in some instances by the use of a proppant coated with a curable resin which will consolidate and cure underground. Phenolic resin coated proppants have been commercially available for some time and used for this purpose. Thus, resin-coated curable proppants may be employed to xe2x80x9ccapxe2x80x9d the fractures to prevent such flow back. The resin coating of the curable proppants is not significantly crosslinked or cured before injection into the oil or gas well. Rather, the coating is designed to crosslink under the stress and temperature conditions existing in the well formation. This causes the proppant particles to bond together forming a 3-dimensional matrix and preventing proppant flow-back.
These curable phenolic resin coated proppants work best in environments where temperatures are sufficiently high to consolidate and cure the phenolic resins. However, conditions of geological formations vary greatly. In some gas/oil wells, high temperature ( greater than 180xc2x0 F.) and high pressure ( greater than 6,000 psi) are present downhole. Under these conditions, most curable proppants can be effectively cured. Moreover, proppants used in these wells need to be thermally and physically stable, i.e. do not crush appreciably at these temperatures and pressures.
Curable resins include (i) resins which are cured entirely in the subterranean formation and (ii) resins which are partially cured prior to injection into the subterranean formation with the remainder of curing occurring in the subterranean formation.
My shallow wells often have downhole temperatures less than 130xc2x0 F., or even less than 100xc2x0 F. Conventional curable proppants will not cure properly at these temperatures. Sometimes, an activator can be used to facilitate curing at low temperatures. Another method is to catalyze proppant curing at low temperatures using an acid catalyst in an overflush technique. Systems of this type of curable proppant have been disclosed in U.S. Pat. No. 4,785,884 to Armbruster and the disclosure of this patent is incorporated by reference in its entirety. In the overflush method, after the curable proppant is placed in the fracture, an acidic catalyst system is pumped through the proppant pack and initiates the curing even at temperatures as low as about 70xc2x0 F. This causes the bonding of proppant particles.
Due to the diverse variations in geological characteristics of different oil and gas wells, no single proppant possesses all properties which can satisfy all operating requirements under various conditions. The choice of whether to use a precured or curable proppant or both is a matter of experience and knowledge as would be known to one skilled in the art.
In use, the proppant is suspended in the fracturing fluid. Tbus, interactions of the proppant and the fluid will greatly affect the stability of the fluid in which the proppant is suspended. The fluid needs to remain viscous and capable of carrying the proppant to the fracture and depositing the proppant at the proper locations for use. However, if the fluid prematurely loses its capacity to carry, the proppant may be deposited at inappropriate locations in the fracture or the well bore. This may require extensive well bore cleanup and removal of the mispositioned proppant.
It is also important that the fluid breaks (undergoes a reduction in viscosity) at the appropriate time after the proper placement of the proppant. After the proppant is placed in the fracture, the fluid shall become less viscous due to the action of breakers (viscosity reducing agents) present in the fluid. This permits the loose and curable proppant particles to come together, allowing intimate contact of the particles to result in a solid proppant pack after curing. Failure to have such contact will give a much weaker proppant pack.
Foam, rather than viscous fluid, may be employed to carry the proppant to the fracture and deposit the proppant at the proper locations for use. The foam is a stable foam that can suspend the proppant until it is placed into the fracture, at which time the foam breaks. Agents other than foam or viscous fluid may be employed to carry proppant into a fracture where appropriate.
Also, resin coated particulate material, e.g., sands, may be used in a wellbore for xe2x80x9csand control.xe2x80x9d In this use, a cylindrical structure is filled with the proppants, e.g., resin coated particulate material, and inserted into the wellbore to act as a filter or screen to control or eliminate backwards flow of sand, other proppants, or subterranean formation particles. Typically, the cylindrical structure is an annular structure having inner and outer walls made of mesh. The screen opening size of the mesh being sufficient to contain the resin coated particulate material within the cylindrical structure and let fluids in the formation pass therethrough.
While useful proppants are known, it would be beneficial to provide proppants having improved features such as reduced flow-back, increased compressive strength, as well as higher long term conductivity, i.e., permeability, at the high closure stresses present in the subterranean formation. Reduced flow-back is important to keep the proppant in the subterranean formation. Improved compressive strength better permits the proppant to withstand the forces within the subterranean formation. High conductivity is important because it directly impacts the future production rate of the well.
It is an object of the present invention to provide proppants coated with fiber-containing polymer.
It is another object of the present invention to provide curable proppants coated with fiber-containing phenol-aldehyde novolac polymer.
It is another object of the present invention to provide precured proppants coated with fiber-containing phenol-aldehyde resole polymer.
It is another object of the present invention to provide methods of using proppant coated with a fiber-containing polymer.
It is another object of the present invention to provide methods of using proppant coated with a fiber-containing polymer.
These and other objects of the present invention will become apparent from the following specification.